Ambient cured flexible fluoroelastomer coatings and coated products

ABSTRACT

The present invention provides an ambient temperature curable fluoroelastomer coatings and coated articles to provide improved elongation, and adhesion to substrates especially flexible elastomeric substrates. The curable coating mixture is a mixture of two parts, part A containing a solution of a fluoroelastomer containing a specified acid number, and the other part B providing a curing component which is a mono-primary amine containing silane or condensate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of co-pending U.S. patentapplication Ser. No. 11/037,678 filed Jan. 18, 2005, which is acontinuation-in-part of U.S. patent application Ser. No. 10/205,178filed Jul. 25, 2002, now U.S. Pat. No. 6,844,412.

FIELD OF INVENTION

The present invention is directed to organic solution coatings which areambient temperature curable and capable of being brushed, dipped orspray-applied to conventional substrates such as rubbery elastomericseals, rings, moldings, housings, rubber-metal composites andflexible-to-rigid thermoplastic substrates. The coatings provideenhanced appearance, resistance to solvents and fuels, and enhancedozone resistance for the underlying elastomer substrate. The coatingsare highly extensible by providing flexible crosslinks as definedhereinbelow. The coatings are especially useful to coat molded rubbergoods, such as pneumatic or non-pneumatic wheels and tires, hoses,belts, rubber bonded metallic mounts, and the like, especially whereused near hot bodies, such as engine blocks coming into contact withfuels and oils.

BACKGROUND OF THE INVENTION

There is a well-known difficulty in obtaining good adhesion betweenelastomeric coatings based on polar elastomers and non-polar rubbersubstrates. Useful elastomeric coatings have been made from such polarelastomers as carboxylated hydrogenated nitrile rubber (XHNBR) orethylene acrylic terpolymers (AEM) in U.S. Pat. No. 6,844,412.

Common examples of nonpolar elastomers as substrates include as naturalrubber (NR), polyisoprene (IR), polybutadiene (BR), styrene butadiene(SBR) and ethylene propylene (EPDM). One known method used to improveadhesion to a nonpolar elastomer substrate has been to treat thesubstrate with a chlorinating or oxidizing acid such astrichloroisocyanuric acid. This treatment is undesirable in manyapplications because the acid residue can cause corrosion to the sprayequipment, dip tanks, and exhaust systems in the application lines andthere is the suspicion that it may initiate corrosion of rubber to metalbonded parts that have been treated prior to coating.

Heretofore, commercial primers such as Chemlok® 254X have been used toenhance adhesion to EPDM, but is of limited use in preparing NR, BR SBR,and blends thereof for adhesion of polar overcoatings. Such nonpolarelastomers are most commonly used in dynamic applications ranging fromtires to vibration isolators and dampers. Additional primers having easeof use and less corrosivity are needed.

Conventional fluoroelastomers as coatings suffer from poor adhesion tomany substrates and insufficient elongation when coated over flexibleelastomers. Existing commercial fluoropolymer coatings delaminate andstress crack after limited flex-testing. Improved fluoroelastomercoatings are sought that provide curing in several hours and sufficientpot life to enable brushing, dipping and spray applications, and providefunctional coatings on a variety of flexible elastomers, especiallynonpolar elastomers.

SUMMARY OF THE INVENTION

The present invention provides a mixture of two parts, one partcontaining a fluoroelastomer and the other part containing a cure agent,and coatings and coated articles therefrom that provide surprisingsubstrate adhesion, improved elongation, and weatherability for avariety of purposes.

In another embodiment, a primer-coat-top-coat system is provided, wherethe first coating is a primer comprising a functionalized film-formingprimer. Preferred primers coating compositions are highly saturatedpolymers reactive with organosilanes, and exhibiting a Tg of less than0° C. Highly saturated means a level of from 0 to 20% unsaturation. Thepreferred curable primer polymer is mixed with from 25 to 150 parts byweight per one hundred parts by weight of priming polymer, of a silanecompound, oligomer, or polymer containing silicone bonded groupscoreactive with the functional group on the primer polymer in thepresence of moisture.

The room temperature curable coating mixture comprises a solution offluoroelastomer in solvent. The fluoroelastomer contains an acidic curesite such that the measured acid number is from 2 to 6 mg base per gramof fluoroelastomer, and preferably from 3 to 5 mg base per gram offluoroelastomer. The curing component is a mono-primary aminosilanecompound, oligomer or polymer, and present at an equivalent ratio ofamine:acid cure site of from 3:1 to 12:1. The mixed parts of polymer andcuring component provide a pot-life of from 1 hour to several hoursdepending on the reactivity of the selected curing components. Thecurable film former and curing component are mixed together at anoverall 4% to 25% solids content. The viscosity can vary depending onthe selected components and is typically less than 20,000 cps(Brookfield LVF) at solids levels of around 10 wt. %. The coating can besprayed, brushed or dipped at these viscosities and allowed to dry toform dry films ranging in thickness (DFT) of from 0.001 to about 0.020in. (0.025-0.51 mm). Typically, multiple layers are applied when higherDFT is desired.

More specifically, the fluoroelastomer coating composition of theinvention comprises a dilute, sprayable coating containing typicallyabout 4 wt. % to 25 wt. % dissolved fluoroelastomer solids in an organicsolvent, in the substantial absence (<100 ppm) of water and less than1000 ppm of free isocyanate groups. No transition metal catalysts, ormetal oxides are used. The primary aminosilane chemically bonds via theprimary amine to cure sites on the fluoroelastomer and the alkoxysilanegroup(s) bond to other alkoxysilane groups bound through another primaryamine at another cure site on the fluoroelastomer when exposed toambient moisture.

In another aspect, the fluoroelastomer coating compositions are appliedover previously applied primer polymers. The most preferred primerpolymers are coatings containing functionalized hydrogenated nitrilerubber, or carboxy-modified ethylene copolymers.

In another embodiment, there is provided a coated elastomer substratehaving two sides such as a molded elastomeric cover, wherein on a sidewhich is to be exposed to fuels, oils, solvents and the like, this sidecontains a thin coating of the fluoroelastomer as a protective layer onthe elastomer. In the case of seals or gaskets, where one side isintended to be exposed to a chemical environment different than theother side, one side of the seal is coated with or formed from ahydrogenated polybutadiane, or other highly saturated non-fluorinatedpolymer and the other side is coated to contain an outer layer of thefluoroelastomer coating composition according to the present invention.In yet another embodiment, the entire seal is made from an elastomerother than HNBR, and is coated entirely with a HNBR primer coatingdisclosed herein, and one side is further coated with a second coatingof fluoroelastomer according to the present invention.

In another specific embodiment, particulate metal-filled emissiveelastomeric coatings are provided which are devoid of vulcanizationchemicals, e.g., peroxides, and accelerators, which are room temperaturecurable, without heat. These provide durable, weatherable, long-termflexible films tightly adhered to the substrate and provide heatdissipation when applied to flexible polymeric substrates, especiallyvulcanized rubber articles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The fluoroelastomers used herein are hydrophobic. By hydrophobic ismeant that at least 80% of the fluoroelastomer is derived from waterinsoluble monomers.

The class of solvent-soluble fluoroelastomers disclosed herein have beenfound to cure to elastomer substrates at ambient temperatures andprovide at least 200% elongation after curing. Such elongationsurprisingly overcomes the limitations in flex-cracking exhibited byconventional fluoroelastomer coatings. The curing of the fluoroelastomeraccording to the present invention provides flexible crosslinks, havingat least 8 intervening atoms linked in a chain between different curesites on the fluoroelastomer. Such a flexible crosslink providessurprising long-term flexing capability, tensile and elongationproperties.

Representative of fluoroelastomers containing acidic cure sites includecarboxylated fluoroelastomers. These materials are believed to cure withthe curing component by formation of bonds between carboxyl groups andmono-primary amino group on the aminosilane in anhydrous conditions.Salt forming reactions are believed to be supplemented by condensationreactions between two salt-bridged curing components and/or to theacidic polymer cure sites. The surprising enhanced tensile strength andelongation properties are believed to occur by the formation of flexiblecrosslinkages containing at least 8 interchain atoms.

The interaction between curing component and functional groups on thecarboxylated fluoroelastomer polymer include electrophile-nucleophileinteractions. Acidic cure sites, e.g., carboxyl cure sites on thefluoroelastomer can be provided by copolymerization of a comonomerbearing a carboxylic acid group or by various known methods formodifying fluoroelastomers by incorporation of acidic functional groupsonto the polymer after polymerization.

The term “functionalized” generally applied to film forming polymers,particularly fluoroelastomers, means (1) that an electrophile,nucleophile, especially an active hydrogen-bearing moiety is part of anethylenic unsaturated comonomer that is copolymerized, or (2) anelectrophile, nucleophile and especially an acidic hydrogen bearingcompound is part of a graftlinking compound graft-linked to a basefluoroelastomer, or film former after- or post-polymerization. Thediscussion below particularly applies to fluoroelastomers, but isequally applicable to primer polymers useful in the invention as primersunder the fluoroelastomer coatings.

The fluoroelastomer cure site can be a comonomer or grafted compoundthat becomes ionically and/or covalently bonded to the polymerstructure, and provides a pendant group capable of reacting with acuring component at ambient temperatures. Terminal functional groups canbe present, although it is critical that sufficient pendant cure sitesare formed or present, such that the fluoroelastomer exhibits an acidnumber of from 2 to 6 mg base per gram of fluoroelastomer.

Incorporation of an acidic hydrogen-bearing functional group or aco-reactive group therewith into a non-functional fluoroelastomer isprovided by converting a functional group-bearing compound into asuitable functional group precursor or by the direct incorporation of asuitable precursor radical when the fluoroelastomer is forming, isformed and in solution or is formed and in the molten state. Arepresentative known post-polymer method includes the “Ene” reaction,whereby an allylic hydrogen transfers to an enophile followed bycoupling between two unsaturated termini, or via free-radical additionacross a dehydrohalogenated repeating unit in solution or in the heatedmolten state.

When the fluoroelastomer is in the molten state, however, means capableof imparting high mechanical shear as are known, such as an extruder, ormill will be used to effect the desired reaction to incorporate thefunctional group or directly incorporate a suitable precursor radical.When the functional group to be converted to a suitable precursor or theprecursor radical is incorporated via techniques such as metallationfollowed by reaction with a suitable electrophile, on the other hand,incorporation of cure site compounds will, preferably, be accomplishedwith the polymer in solution.

A variety of post-polymerization functionalization techniques are knownwhich provide heretofore non-functional addition polymers withnucleophilic, or electrophilic crosslinking cure sites for use in thepresent invention. Hydroxyl groups are useful functional groups foreffecting the crosslinking reactions with curing components used herein.U.S. Pat. No. 4,118,427 discloses hydroxyl-containing curable liquidhydrocarbon prepolymers by ozonizing a high molecular weight saturatedhydrocarbon polymer such as polyisobutylene or ethylene-propylenerubber, followed by reducing the ozonized material; e.g., by usingreducing agents such as diisobutyl aluminum hydride, to form thehydroxyl-containing polymer.

A partial listing of nucleophilic and/or acidic hydrogen functionalgroups that can be incorporated on the fluoroelastomer and coreactivewith electrophilic group-substituted curing components or hydrolyzablecuring agents are, hydroxy, mercapto-, isocyanato-, amino-, phenolic-,and carboxyl-groups. Exemplary electrophilic groups incorporated on thefluoroelastomer and coreactive with nucleophilic group-substitutedcuring components are alkyl halide-, benzyl halide, allyl halide-,ester-, ethers-, anhydride-groups, and the like. When thefluoroelastomer contains a pendant nucleophilic group, the correspondinggroup provided on at least one valency of the silicone atom of thesilane curing component can also include an alkoxy-, hydroxy-,mercapto-, isocyanato-, amino-, phenolic-, glycido-, carboxyl-,oxirane-, benzyl halide-, allyl halide-, alkyl halide-, ester-, ethers-,and/or anhydride-group.

A graft-functionalized fluoroelastomer embodiment film former utilizedherein is the reaction product of a fluoroelastomer polymer and agrafting agent which contains a graft linking group which covalentlybonds to the fluoroelastomer, and at least one activehydrogen-containing group, including but not limited to hydroxyl, thiol,or carboxyl groups that undergo bond formation to one of the reactivegroups of the curing component. The graft-modified fluoroelastomer partA is combined with the curing component part B by simple admixture, andused within the expected pot life prior to gellation to coat thesubstrate.

The representative fluoroelastomers used herein include polymers derivedfrom one or more fluorinated monomers. The preferred fluoroelastomersused herein are derived from such monomers as vinylidene fluoride, andhexafluoropropylene and are commercially available from a number ofsuppliers. Example fluoroelastomers result from combinations of two ormore fluorinated monomers including 1,1-dihydroperfluorobutyl acrylate;copolymers of vinylidene fluoride and chlorotrifluoroethylene;vinylidene fluoride and hexafluoropropylene; vinylidene fluoride andhydropentafluoropropylene; tetrafluoroethylene and propylene; andterpolymers of vinylidene fluoride, hexafluoropropylene, andtetrafluoroethylene; vinylidene fluoride, tetrafluoroethylene andperfluorovinyl ether; vinylidene fluoride, tetrafluoroethylene, andpropylene; vinylidene fluoride and hydropentafluoropropylene andtetrafluoroethylene. The most preferred fluoroelastomer modifiedaccording to the invention are commercially available provided the acidnumber is from 2 to 6 mg base per gram of fluoroelastomer. CertainViton® copolymers of vinylidenefluoride and hexafluoropropylene, or aterpolymer of vinylidenefluoride, tetrafluoroethylene, andhexafluoropropylene are believed to contain sufficient acid numbers soas to be suitable herein. Other suitable fluoroelastomers are availablefrom Dyneon under the FLUOREL® mark, and from Ausimont under theTECHNIFLON® mark.

If the fluoroelastomer exhibits an acid number below about 2 mg KPH pergram of polymer, the coatings do not completely cure and do not developsufficient tensile strength. If the ratio of equivalents of primaryamine to equivalents of acid cure sites is less than about 3:1, the sameincomplete curing and insufficient film toughness arises. If the acidnumber exceeds 6 mg base per gram of polymer, the film exhibitsinsufficient elongation, and flex-cracking is detrimentally affected.The mono primary aminosilane curing agent is critical. Secondaryaminosilanes do not exhibit ambient temperature curing.

A copolymerizable comonomer is preferably a monocarboxylic, but can bepolycarboxylic acid. Preferred carboxyl comonomers contain from 3 toabout 8 carbon atoms. Examples of such preferred comonomers are acrylicacid, methacrylic acid, ethacrylic acid, β,β-dimethylacrylic acid,crotonic acid, 2-pentenoic acid, 2-hexenoic acid, maleic acid, furmaricacid, citraconic acid, mesaconic acid, itaconic acid,3-butene-1,2,3-tricarboxylic acid, and the like. The most preferredcarboxyl comonomers are the monocarboxylic acid monomers such as acrylicacid, methacrylic acid, itaconic acid, and the like.

Poly(olefin/acrylic ester/carboxylate) copolymers useful as primerpolymers herein are thermoplastic in the uncured state and are suitablyflexible for use as part of the primer coating. These are principallycopolymers produced by polymerizing at least one α-olefin with at leastone C₁-C₁₈ alkyl (meth)acrylate and a minor amount of an unsaturatedprotic functional group-bearing comonomer that is accessible to formcrosslinks with such materials as polyisocyanates, carbodiimides, andother curing agents. Functional group bearing comonomers can comprise anethylenic unsaturated group and a group bearing an acid, hydroxy, epoxy,isocyanate, amine, oxazoline, diene or other reactive groups. In theabsence of such functionalized monomer, crosslinking sites can begenerated in an a-olefin-ester copolymer, e.g. by partial hydrolysis ofpendant ester groups. Suitable a-olefins for polymerization of sucholefin copolymer film-forming elastomers include ethylene, propylene,butene-1, isobutylene, pentenes, heptenes, octenes, and the likeincluding combinations. C₂-C₄ α-olefins are preferred, and ethylene ismost preferred.

The alkyl or alkoxy(meth)acrylate acids and esters are exemplaryfunctionalized comonomers for incorporation into α-olefin primerpolymers. Concrete examples of alkyl groups are a methyl group, ethylgroup, n-propyl group, isopropyl group, n-butyl group, isobutyl group,sec-butyl group, t-butyl group, pentyl group, hexyl group, octyl group,2-ethylhexyl group and decyl group; cycloalkyl group such as cyclopentylgroup and cyclohexyl group; aryl group such as phenyl group and tolylgroup; and aralkyl group such as benzyl group and neophyl group.Examples of alkoxy groups include methoxy group, ethoxy group, n-propoxygroup, isopropoxy group, n-butoxy group, isobutoxy group, sec-butoxygroup, t-butoxy group, pentoxy group, hexoxy group and octoxy group.

Suitable alkyl or alkoxy (meth)acrylates for copolymerizing with theα-olefin include methyl acrylate, ethyl acrylate, t-butyl acrylate,n-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethylmethacrylate, n-butyl methacrylate, 2-ethyle-hexy acrylate, methoxyacrylate, ethoxyethyl acrylate, methoxyethyl acrylate, acrylamide, andmethacrylamide, and the like or a mixture thereof. Specific examples offunctional ethylenically unsaturated monomers which are copolymerizablewith the olefin monomers are: unsaturated carboxylic acids such asacrylic acid, methacrylic acid, itaconic acid and maleic acid and saltsthereof, optionally in combination with alkyl esters of unsaturatedcarboxylic acids such as methyl acrylate and butyl acrylate.

Other comonomers which contain a functional acid, hydroxy, epoxy,isocyanate, amine, oxazoline, diene or other reactive functional groupinclude the diene monomers, such as non-conjugated dienes such asalkylidenenorbornene, alkenylnorbornene, dicyclopentadiene,methylcyclopentadiene and a dimer thereof and conjugated dienes such asbutadiene and isoprene. Examples of the dihydrodicyclopentadienylgroup-containing (meth)acrylate include dihydrodicyclopentadienyl(meth)acrylate and dihydrodicyclopentadienyloxyethyl (meth)acrylate.

A preferred olefin/acrylic ester copolymer useful as a primer polymerincorporates unsaturated carboxylic acid monomer units, such from(meth)acrylic acid or maleic acid, anhydride units, e.g. derived frommaleic anhydride or partial ester units, e.g. mono ethyl maleate. In apreferred embodiment the polymer is a terpolymer of ethylene, C₁-C₄alkyl acrylate and an carboxylic monomer unit; more preferably suchterpolymer comprises at least about 30 mole percent of ethylene, about10 to about 69.5 mole percent mono ethyl maleate. In all cases it ispreferred that the α-olefin acrylate rubber be essentiallynon-crystalline and have a glass transition temperature (Tg) below about20° C. Ethylene-carboxylate copolymers are available commercially underthe VAMAC® mark.

The primer polymers suitable for making a primer coating can be selectedfrom various polymer blends, alloys, dynamically vulcanized polyolefins,composites of maleated addition polymers based on polyethylenes, such asmaleated polypropylenes, maleated styrene-ethylene-butene-styrene-blockcopolymers, maleated styrene-butadiene-styrene block copolymers,maleated ethylene-propylene rubbers, and blends thereof can be utilizedas the functionalized film-forming elastomer in accordance with theinvention.

The most preferred functionalized film forming primer polymers, appliedbefore the fluoroelastomer coating have a T_(g) of less than 0° C. andare selected from carboxylated hydrogenated nitrile rubber and carboxymodified ethylene copolymers (sold under the tradename of Vamac® byDuPont).

The mono-primary aminofunctional silane curing agents used hereininclude those having the structure (B)

wherein R, R¹, R² and a are as previously defined for (A); and R⁵ isselected from the group consisting of hydrogen, monovalent aliphaticradicals having from 1 to 8 carbon atoms, monovalent cycloaliphaticradicals having from 4 to 7 ring carbon atoms, phenyl, alkaryl radicalshaving 6 nuclear carbon atoms and containing one or more substituentalkyl groups having from 1 to 4 carbon atoms, and -R⁶—NH—R⁷, wherein R⁶is selected from the group consisting of divalent aliphatic,cycloaliphatic and aromatic radicals having from 1 to 20 carbons, therebeing preferably at least two carbon atoms separating any pair ofnitrogen atoms, with R⁶ being preferably an alkylene group of 2 to 9carbon atoms; and R⁷ being the same as R⁵ and preferably is hydrogen.

Representative curing agents which are mono-primary amines include thoseselected from γ-aminopropyltrimethoxysilaneγ-aminopropyltriethoxysilane, methylaminopropyltrimethoxysilane,γ-aminopropyltripropoxysilane, γ-aminoisobutyltriethoxysilane,γ-aminopropylmethyldiethoxysilane, γ-aminopropylethyldiethoxysilane,γ-aminopropylphenyldiethoxysilane, δ-aminobutyltriethoxysilane,δ-aminobutylmethyldiethoxysilane, δ-aminobutylethyldiethoxysilane,γ-aminoisobutylmethyldiethoxysilane,N-methyl-g-aminopropyltriethoxysilane,N-phenyl-γ-aminoisobutylmethyldiethoxysilane,N-ethyl-δ-aminobutyltriethoxysilane,N-γ-aminopropyl-γ-aminopropyltriethoxysilane,N-β-aminoethyl-γ-aminoisobutyltriethoxysilane,N-γ-aminopropyl-δ-aminobutyltriethoxysilane,N-aminohexyl-γ-aminoisobutylmethyldiethoxysilane,methylaminopropyltriethoxysilane, γ-aminopropylmethoxydiethoxysilane, oras depicted as 3-aminopropyl triethoxy silane, 3-aminopropyl trimethoxysilane, 3-aminopropyl methyl dimethoxysilane or 3-aminopropyl methyldiethoxy silane, N-(2-aminoethyl)-3-aminopropyl trimethoxy silane,condensed aminoalkyl silanes such as bis(3-aminopropyl) tetramethoxy ortetraethoxy disiloxane NH₂(CH₂)₃—Si(OCH₃)₂—O—(CH₃O)₂Si—(CH₂)₃NH₂,polyglycol ether-modified aminosilanes such as that sold under theTrademark “Dynasylan 121” and triamino functional propyl trimethoxysilanes such as “Dynasylan TRIAMO” available from Huls AG.

The curing component must contain only one primary amine and at leastone hydrolyzable group, preferably up to 4 hydrolyzable groups. Informing crosslinks between the fluoroelastomer cure sites, the silanecouples to the fluoroelastomer in the absence of water by what isbelieved to be an initial ionic bond to the acidic cure-site, andextends from the fluoroelastomer via FK-O⁻—NH⁺—R—Si—OR, where FK is thefluoroelastomer at the acidic cure site, and R is any divalenthydrocarbyl moiety containing any of C, O, N and S moieties. Linking ofadjacent Si—OR groups is believed to proceed by moisture-inducedcondensation. There are myriad hydrocarbyl groups provided by the manyknown organosilanes representing the crosslink chain, and readilyapparent from the several examples provided herein. The two co-reactivecrosslinkable groups provide a total of at least 8 atoms bridging thefluoroelastomer, and preferably from 10-16 linking atoms between thecrosslinked polymer cure sites. The preferred hydrocarbyl groups areC₂-C₆ substituted or unsubstituted alkylene groups. The preferredhydrolyzable groups bonded to each silicone atom couple to each otherand are C₁-C₄ alkoxy groups.

The term “hydrolyzable group” means any group attached to the siliconwhich is hydrolyzed in the presence of moisture. The hydrolyzablesilicone bonded groups include, halogen atoms such as F, Cl, Br or I;alkoxy groups of the formula —OY when Y is any hydrocarbon orhalogenated hydrocarbon group such as methyl, ethyl, isopropyl,octadecyl, allyl, hexenyl, cyclohexyl, phenyl, benzyl, beta-phenylethyl,and hydrocarbyl ethers such as 2-methoxyethyl, 2-ethoxyisopropyl,2-butoxyisobutyl, p-methoxyphenyl or —(CH₂CH₂O)₂CH₃; or any N,N-aminoradical such as dimethylamino, diethylamino, ethylmethylamino,diphenylamino, or dicyclohexylamino. Not preferred are amino radicals Xsuch as NH₂, dimethylamino, diethylamino, methylphenylamino ordicyclohexylamino; any ketoxime radical of the formula —ON═CM₂ or—ON═CM′ in which M is any monovalent hydrocarbon or halogenatedhydrocarbon radical and M′ is any divalent hydrocarbon radical bothvalences of which are attached to the carbon, such as hexylene,pentylene or octylene; ureido groups of the formula —N(M)CONM″₂ in whichM is a hydrocarbon or halohydrocarbon radical and M″ is H or any of theM radicals; carboxyl groups of the formula —OOCMM″ in which M and M″ aredefined above or halogenated hydrocarbon radical, or carboxylic amideradicals of the formula —NMC═O(M″) in which M and M″ are defined above.X can also be the sulfate group or sulfate ester groups of the formula—OSO₂(OM) where M is defined above hydrocarbon or halogenatedhydrocarbon radical; the cyano group; the isocyanate group; and thephosphate group or phosphate ester groups of the formula —OPO(OM)₂ inwhich M is defined as above.

The natural color of the preferred fluoroelastomer coatings of thepresent invention are clear in the absence of added pigments such ascarbon black. Color and/or opacity can be obtained with known pigmentgrinds according to conventional coating formulation techniques.Particulate metal powders are useful for reflective properties. The term“particles” is inclusive of irregular shapes, granular shapes, leafyshapes or complex assorted shapes. Heat reflective pigments arecommercially available in many forms, as fine-grain solids or asleafy-shaped flakes. These are available as dispersions or pastes insolvent, e.g., mineral spirit. Flakes derived from finely divided vapordeposited films are suitable. Metallic particles of a particle sizeaverage of 5 to 25 μm employed at a level of at 10 to 100 parts byweight per 100 parts by weight of fluoroelastomer when cast in a thinfilm of 5 mils (0.01 cm.) provide effective radiant energy emmissivityand yet provide sufficient flex-fatigue resistance in the coating so asto not undergo stress-cracking. Metal particles having an averageparticle size of 25 to 100 microns must be employed at a level of atleast 20 parts and up to 150 weight parts per 100 parts by weight offluoroelastomer to provide sufficient radiant heat emissivity withoutstress cracking. Preferred aluminum particles are flakes of a size suchthat 99.9% pass through 325 mesh screen, i.e., a diameter of less thanabout 45 microns, most preferably from 8 and 35 and especially from 10and 20 microns in average particle size.

For the purposes of the present invention, the term solvent can broadlybe defined as a free-flowing liquid carrier capable of dissolving ormaintaining the organic components in a substantially dispersed state,and preferably in solution. Preferred solvents include water basedlatexes and/or non-HAP (Hazardous Air Pollutant) or non-VOC, or non-HAP,non-VOC organic solvents.

Non-HAP solvents include methyl acetate, n-butyl acetate, t-butylacetate, acetone, ethyl acetate, isopropyl acetate, isobutyl acetate,tetrahydrofuran, n-methyl pyrrolidone, aliphatic hydrocarbons such asheptane, dimethylformamide, diisobutyl ketone (DIBK), methyl isoamylketone, monochlorotoluene, para-chlorobenzotrifluoride (PCBTF), and vm&pnaphtha. A combination of acetone and DIBK is the preferred non-HAPsolvent mixture. Acetone, methyl acetate, andpara-chlorobenzotrifluoride (PCBTF) alone or in any combination are thepreferred solvents for HAP, and VOC compliant coatings. Among the HAPsolvents which are photochemically reactive in the atmosphere arehexane, xylene, toluene, MEK, and MIBK. Toluene, xylene, MEK and MIBKare the preferred solvents when HAP and VOC compliance is not critical.

On a weight percentage basis, the nonvolatiles are generally present atfrom about 3 to about 30% wt. percent with the remainder being solvent,and preferably from about 5 wt % to about 15 wt. % nonvolatiles.

Coated Articles

A myriad of products comprising moldings made from elastomers such asHNBR, natural rubber, polyisoprene, polybutadiene, styrene butadiene andethylene propylene rubber are coated according to the invention.Included are the elastomeric products which are designed to flex andbend, distort, and/or dampen forces including absorbing torque orrepeated vibration, or the articles may bit be flexed in service, butare exposed to fuels, oils and the like during their service life asutilized in numerous industrial applications. Specific examples arehoses, seals, gaskets, mounts, such as engine mounts, dampers andinsulating devices, to name a few. As molded parts, like rubber hoses,boot coverings, housings, belts, various mounts, shrouds, seals,grommets, washers, gaskets, spacers, covers, made from more generalpurpose rubbers or thermoplastic elastomers or of the thermosetting(vulcanized) rubber materials. Adhesion of the coatings to thesesubstrates is essential as well as obtaining cured physical properties,e.g. toughness and elongation. The coatings, on pneumatic tires must becapable of 100% elongation, and exhibit no distortion. Preferably thecoatings recover completely when extended up to 200% elongation withoutcracking or delaminating from the flexible elastomer substrate. Thefluoroelastomer coatings, according to the invention, exhibit improvedelongation of 200%, and preferably 300%+/−50%, and tensile strength of600 psi or more, as tested according to ASTM-D412 on cured, unsupportedcoating films.

The following nonlimiting examples illustrate the comparative effects ofcertain technical requirements according to the invention.

EXAMPLES

To measure acid number an elastomer is dissolved in acetone or a 1:1acetone/MIBK solution and then titrated with 0.01 N sodium hydroxide toa phenolphthalein endpoint. The examples below demonstrate that afluorocarbon polymer with an acid number of about 1 (+/−0.02) mg baseper gram of polymer does not cure using a mono-primary aminosilane or itcures poorly to give unacceptably low tensile strength and lowelongation. Unacceptable tensile strength is below 600 p.s.i., andunacceptable elongation is below 200%. The Examples below illustratethat the minimum ratio of equivalents of primary mono amino silane tocure site acid equivalents is 3:1. As the equivalent ratio is increasedup to 12:1, tensile strength improves without unacceptable loss ofelongation. Above 12:1 equivalent ratio, strength drops and the claritydecreases. Example 1 2 3 4 5 6 7 8 9 Viton ® A-100 100 100 100 — — — — —— Dai-el ® G902 — — — 100 — — — — — Dai-el ® G704BP — — — — 100 100 100100 — Technoflon ® N535 — — — — — — — — 100 APTES* 4.0 8.0 10.0 10.0 3.06.0 10.0 15.0 10.0*3-aminopropyletriethoxysilane

Tensile strength (psi) DNC DNC DNC DNC DNC 915 1495 1170 1480 Elongation(%) nm nm nm nm nm 905 795 400 590 Acid Value 0.99 0.99 0.99 0.24 3.883.88 3.88 3.88 4.02 Clarity nm nm nm nm nm clear clear cloudy clear(DNC = did not cure)Nm = not measurable

Primer Coating Examples

In experiments to test coating adhesion to natural rubber, a primerconsisting of a solvent, and carboxylated elastomers, e.g., carboxylatedisoprene resin, carboxylated NBR and carboxylated polyethylene wereformulated according to the teachings in copending app. Ser. No.10,205,178, incorporated herein by reference as if entirely containedherein. The formulations in solvent contained from 25-150 phr of aprimary monoaminoalkoxysilane or blend of primarymonoaminoalkoxysilanes. Coatings applied and allowed to stand overnightexhibited good adhesion to natural rubber. It was observed that neitherthe aminosilane nor the carboxylated resins were individually effectivein bonding to natural rubber. Non-amino silanes were evaluated and foundto be ineffective as was a maleic anhydride adducted polybutadiene.

The priming of rubber substrates successfully replaced the conventionalpretreatments using Chemlok® 7701 or 7707

Surprisingly when the above primers were applied to nonpolar elastomersubstrates, the fluoroelastomer coatings according to the presentinvention exhibited outstanding adhesion and flex resistance. Thefluoroelastomer coating improved the fuel and solvent resistance of themolded natural rubber parts.

1. An ambient temperature curable coating composition in a mixture of2-parts providing a non-volatiles content of from 4% to 25% by weight,and comprising a part A which comprises an organic solvent andfluoroelastomer dissolved therein, said fluoroelastomer containingcrosslinkable sites in quantitative amount according to an acid numberof from 2 to 6 mg base per gram, and a part (B) which comprises anorganosilane comprising hydrolyzable groups and a mono primary amine,wherein the ratio of amine equivalents in part B to acid equivalents inpart said A ranges from 3:1 to 12:1 and wherein a cured unsupported filmfrom said coating exhibits at least 200% elongation.
 2. The coating ofclaim 1 wherein said fluoroelastomer has an acid number of from 3 to 5mg base per gram.
 3. The coating of claim 1 wherein said film formingpolymer is a hydrogenated random or block diene copolymer having amolecular weight of about 20,000 to 200,000.
 4. The coating of claim 1wherein said acid equivalents are from functional groups selected from asulfonic acid, a sulfenate, a sulfinate, chlorosulfonic acid, acarboxylic acid, a dicarboxylic acid, a partial ester of a carboxylicanhydride, and cyclic imides of dicarboxylic acids.
 5. The coating ofclaim 1 wherein said acid equivalents are provided from anα,β-unsaturated carboxylic acid.
 6. A coating wherein said curing isselected from the group consisting of aminopropyltriethoxy, aminopropyltrimethoxy silane, aminoethylaminopropyltriethoxy,aminoethylaminopropyltrimethoxy silane, 3-aminopropyl triethoxy silane,3-aminopropyl trimethoxy silane, 3-aminopropyl methyl dimethoxysilane or3-aminopropyl methyl diethoxy silane, N-(2-aminoethyl)-3-aminopropyltrimethoxy silane, N-methyl-3-aminopropyl trimethoxy silane,N-phenyl-3-aminopropyl trimethoxy silane, bis(3-aminopropyl)tetramethoxy and bis(3-aminopropyl) tetraethoxy disiloxane.
 7. Amulti-layer coating on a flexible elastomer, comprising a primer layerand a top coating, wherein the primer layer comprises a film formerhaving a Tg of less than 0° C. which is cured with from 25 to 150 phr ofan organosilane, and wherein said top coating comprised thefluoroelastomer coating according to claim
 1. 8. A multi-layer coatingaccording to claim 7 wherein the primer coating comprises a polymerselected from the group consisting of carboxylated polyisoprene,carboxylated NBR, and carboxylated polyolefin.
 9. A coated articlehaving first and second surfaces, adhered to said first surface is acured coating comprising a film of carboxylated NBR, carboxylatedisoprene, or carboxylated ethylene copolymer, cured with from 25 to 150phr of an organosilane, and on said second side is adhered a curedcoating comprising the fluoroelastomer coating composition according toclaim
 1. 10. A coated article according to claim 9 selected from a seal,gasket, and a mount.
 11. A coated article according to claim 1 whereinsaid substrate is a molded article from an elastomer selected from HNBR,natural rubber, polyisoprene, polybutadiene, styrene butadiene andethylene propylene rubber.